206 SKELETON OF THE KANGAROO. 



scanty supply of perennial streams ; the torrents of the 

 brief periods of rain are reduced to detaclied pools in the 

 dry season, and these are parched up in the long droughts, 

 leaving hundreds of miles of the country devoid of sur- 

 face water. If, then, the parent herbivore could traverse 

 the required distance to quench its thirst, or satisfy its 

 hanger, the tender young would be unable to follow the 

 dam. A modification of the procreative process has ac- 

 cordingly been superinduced, which characterizes the 

 Australian mammals; the young are prematurely brought 

 forth of embryonic size and helplessness, and are trans- 

 ferred to a pouch of inverted skin, concealing the udder ; 

 and in this marsupium, as in a well-stored vehicle, they 

 are easily transferred by the parent to any distance to 

 which the climatal conditions may compel her to migrate. 

 The economy of this portable nursery, the requisite ma- 

 nipulation of the suckling young therein suspended from 

 the teat, demand a certain prehensile power of the fore- 

 limbs, a freedom of the digits, with some opposable faculty 

 in them, and the possession of so much sense of touch as 

 would be impossible were the digit to be incased in a 

 hoof; the horny matter is accordingly developed only on 

 the upper surface of the finger-end, and is in the form of 

 a claw. But the unguiculate pentadactyle extremity — 

 though a higher grade of structure in the progress of 

 limbs — is not suited for the exigences of the herbivore, 

 and would have appeared utterly incompatible with an 

 existence dependent on grazing in wild pastures, had we 

 argued from knowledge restricted to the forms and struc- 

 tures of the hoofed herbivores of the Europaeo- Asiatic, 

 African, and American continents. How, then, it may 

 be asked, is this difl&culty overcome in the case of a graz- 

 ing animal, necessarily a marsupial, and consequently an 



